MEG II 実験のための陽電子タイミングカウンターの開発

PSIでのハイレートビーム試験

Development of Positron Timing Counter with SiPM for MEG-II Experiment

Beam Test Result in the high rate environment at PSI

西村美紀 （東大）他 ＭＥＧコラボレーション

日本物理学会 2015年 第７０回年次大会早稲田大学 早稲田キャンパス

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Contents

• Physics Motivation

• MEG II Experiment

• Positron Timing Counter

• Beam Test ＠PSI (←MEG II Site)

• Status & Schedule

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Physics Motivation• Search for cLFV (charged

lepton flavor violation), μ+ →e+γ decay– Forbidden in the SM

– Sizable branching ratio is expected by many bSMs

• Most stringent upper limit of the branching ration is set by the MEG experiment;

𝟓. 𝟕 × 𝟏𝟎−𝟏𝟑 90% C.L. (Phys. Rev. Lett. 110(2013) 201801)

Already started exploring bSM region!

3Upgrade MEG experiment (MEG II)

An example, the expectation of the BR from SU(5) SUSY-GUTM. Cannoni et.al., Phys. Rev. D 88, 075005 (2013)

Mass of right handed neutrino

What should we measure?

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We should measure

Timing・Position・Momentum precisely.

Accidental BGDominant

Physics BG

Signal Background

Xenon CalorimeterSiPM readout

Pixelated Positron Timing Counter (TC)

Radiative Decay Counter

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Stereo wire drift chamberTracking the positron till near TC

γµ twice higher

beam intensity （stopping µ ~7×107）

sensitivity 𝟒 × 𝟏𝟎−𝟏𝟒

中浦 24aDL11

澤田・家城・小川(21pDK4,5,6)

This talk, and next talk 吉田(24aDL10)

Pixelated Positron Timing Counter

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𝑵𝒉𝒊𝒕 = ~9

target

𝑵𝒉𝒊𝒕 = ~9

× 512

4 c

m o

r 5 c

m

12 cm

Pixelated Positron Timing Counter

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4 cm

or

5 cm

12 cm

Thickness 5 mm

6 SiPMs in series at the both endAdvanSiD (Italy) 3x3 mm2, 50 um

Fast Plastic ScintillatorBC422, rise time 0.35 nsattenuation length 8 cm

PCB

Cable（RG178）Non-magnetic

PCB

Back planeLong PCB ~80 cmMulti layer, coaxial like

Principle

• Many hits– Averaging timing information, final TC

resolution improve.（We already prove the principle in the other beam tests.）

• Pile Up Reduction– Beam intensity becomes higher in MEG II

• Able to track the positron 8

Resolution vs. # of hit counters

Number of hit counters (MC)MEG I: 80 cm scintillator bar （with 2 PMTs） ×15

MEG II: 12 cm scintillator counter ×250

σsingle = 65 ps*contribution from DC; 75 ps

σsingle = 65 ps *contribution from DC; 11 ps

σ ~30 ps

𝜎2𝑡𝑜𝑡𝑎𝑙 𝑁ℎ𝑖𝑡 =𝜎2𝑠𝑖𝑛𝑔𝑙𝑒𝑁ℎ𝑖𝑡

+𝜎2𝑖𝑛𝑡𝑒𝑟−𝑐𝑜𝑢𝑛𝑡𝑒𝑟

𝑁ℎ𝑖𝑡+𝜎2𝑀𝑆 𝑁ℎ𝑖𝑡

~5 ps~30 ps

Status • Prototype test

– Good single counter performance was already demonstrated.

– Multiple hit scheme was proved by beam tests in a quiet environment.• Positron beam at BTF in Italy• 50 Hz, 1-2 positrons in a bunch

• Calibration (next talk)– Laser– Michel positron

• Software development– Timing reconstruction method

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BEAM TEST

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Motivation• Previous beam test was

conducted at low positron rate. (~50 Hz)

• However in MEG II , TC will suffer high rate positron backgrounds (>~45MeV, <~180kHz)

Operate TC in this high rate environment.

Length [cm]Average Rate

(kHz)Highest Rate

(kHz)12 70.2 174

From MC study (height is 4 cm)

muon normal decay

Set up

Back plane

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We had to use scintillators and SiPMs of bad performance due to quality control issues of vendors.

Muon BeamπE5 beam line at PSI

~108 Hz DC beam

2 small counters5x5x5 mm3, 1 MPPC

Teflon wrapping

Time Reference

Michel PositronNormal decay from stopping muon

The same as real background

We can get time uniform background.

8 Prototype Counters6 for 120x40x5 mm3, 2 for 120x50x5 mm3

AdvanSiD w/o new technology -> w/ new technology

Scintillator BC422 -> EJ232

Reflector 3M Mylar film, light shielded

Collimator

Digitizer (DRS)

1.6 GHz

Coincident Trigger

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Prototype Counters

Reference Counters

Positrons

Back Plane

Counter ACounter B

Counter C

0

50

100

150

200

250

300

350

0 1 2 3 4 5 6 7 8 9H

it R

ate

(kH

Z)

Counters

Michel low Michel expected Michel high

A

Rate We adjusted the beam slit for rate scanning (3 points)

Three hit rate conditions

• Low rate; 17.8 – 64 kHz

• Expected rate; 53 – 166 kHz

• High rate; 89 – 290 kHz

5 cm height

Expected rate in MC

Length [cm]Average Rate

(kHz)Highest Rate

(kHz)12 70.2 174

From MC study (height is 4 cm)

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B C D E F G HA

Analysis• Single Counter

Timing (𝑡1 + 𝑡2)/2

Position 𝑥 = 𝑣𝑒𝑓𝑓(𝑡1 − 𝑡2)/2

• Clustering– Clustering based on

reconstructed time.– Cut the pile up hits

• Tracking– Using reconstructed

position– Far hits from

trajectory are removed.

Notice: There is trade-off b/w strict cut and efficiency.We should optimize the parameter for final analysis

Single Counter Resolution

Small degradation of the resolutions from higher rate can be observed.

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Back plane seems to pick up noise

A

B

C

D

E

F

G

H

Overall Resolution

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Only small degradation of the resolution up to the expected hit rate.

N = 2

N = 3

N = 4N = 5N = 6

N = 8

N = 7

(*Average)

Overall Resolution

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• The difference becomes larger in higher rate. However the difference is not so effective especially b/ lower rate and expected rate.

• At n = 8, the overall resolution is ~35 ps.• This number will improve with final counters because the

single resolutions will improve from ~90 ps to ~65 ps.

Schedule & Status In this beam test, we demonstrated good performance in high rate environment expected in the experiment. It is time to construct and test the final detector.

• Construction– SiPMs were already delivered and tested.– Support structure is under construction.– SiPMs array will be tested soon.– Counter mass production and mass test

• TC DS engineering run is planned in Dec.– Down stream of TC will be installed and tested with the muon beam.– Number of readout electronics channels are limited. (Half of the DS

TC channels can be readout at one time)

• Next spring TC US will be installed.• Summer in 2016, the engineering run will start.• Following the engineering run, physics run will start.

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Summary

• The MEG II experiment will search for μ→eγ decay at a unprecedented sensitivity of 4x10-14.

• The Pixelated Positron Timing Counter of 30 ps resolution is in preparation.

• We conducted a beam test with prototype counters under the MEG II beam condition.– We obtained the excellent resolution of 35 ps with 8

counters.– The final performance of TC will be better by using

better counters.

• Preparation for TC DS engineering run in Dec. is underway.

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BACK UP

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Pileup Study

• Michel positron (4π, all momentum)• Hit rate of Michel positron depends on position.

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The bins correspond to the each pixel.

90x30x5 pixels

The highest rate position

Final Performance

• We could measure the final prototype counters. (only two counters)• The TC performance improved.• In the final detector better performance is expected.

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Time line

• BeamWe lost ~6 days for the investigation.

Sun Mon Tue Wed Thu Fri Sat

12 13 14 15 16 17 18

19 20 21 22 23 24 25

26 27 28 29 30 31 1

2 3 4 5 6 7 8

9 10 11 12 13 14 15

16 17 18 19 20 21 22

TC set up

Beam investigation

Beam tuning

Beaminvestigation

Beam investigation

Michel

Data taking

WD test

Beam tuning for Positron Mott

Beam shat down

Michel Beam tuning

TC set up delivered

TC alignment

ATAR beam time

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